By Tom Gibson, P.E.
Hydrogen seems like an ideal fuel, but a feasibility analysis reveals the factors that will determine its success.
Hydrogen has the highest energy density by weight of any fuel. So it only makes sense that compressed hydrogen gas is being considered as a carbon-free replacement for diesel in heavy-duty trucks. A hydrogen-fueled truck fleet would substantially reduce greenhouse gas emissions and other pollutants from the freight industry.
Before that can happen, engineers need to work out the optimum technology to make hydrogen economical as well as clean.
“Hydrogen technology is currently expensive because of relatively low deployment. Once it is more common, the refueling infrastructure cost should decrease significantly. Hydrogen refueling infrastructure can be implemented immediately. It has the potential to be very economical,” said Wahiba Yaici, a research scientist at the CanmetENERGY Research Centre, Natural Resources Canada in Ottawa, Ontario.
The cost of hydrogen itself looms as the main element in the acceptance of compressed hydrogen. It is rarely found isolated and must go through processes to extract it. Each hydrogen production method involves separating hydrogen from some compound. Common extraction processes include natural gas reforming, biomass gasification, and coal gasification. So-called green hydrogen is produced by using renewable energy such as solar and wind to power the electrolysis of water or other hydrogen-making reactions.
Generally, green hydrogen is more expensive than hydrogen produced by other methods, but it also has the advantage of producing zero carbon emissions.
Storing hydrogen is another issue. Hydrogen at a fueling station can be stored either as a gas or a liquid. Storing hydrogen as a gas typically requires high-pressure tanks at 5,000 to 10,000 psi. Storage as a liquid requires cryogenic temperatures, as the boiling point of hydrogen at one atmosphere pressure is −252.8 °C (-423 °F).
The main options for onboard hydrogen storage in vehicles are gaseous storage at 5,000 psi or at 10,000 psi in thermally insulated tanks or in materials with superior chemical properties, like metal hydrides or organic compounds,” Yaici said.
To address the hydrogen refueling scenario for big trucks, Yaici teamed with Michela Longo, an associate professor at the Politecnico di Milano in Italy, to write a paper, “Hydrogen Gas Refueling Infrastructure for Heavy-Duty Trucks: A Feasibility Analysis,” for the ASME Journal of Energy Resources Technology. Both authors are mechanical engineers. The pair chose to pursue hydrogen because it can be used as fuel in modified internal combustion engines (ICEs), reducing the need to purchase new vehicles. Existing diesel engines can easily be modified to burn hydrogen. This is in contrast to the fuel-cell-powered vehicles most people think of when talking hydrogen; fuel cells combine hydrogen and oxygen, yielding water, heat, and electricity used to power an electric motor that drives the vehicle.
This paper investigates the feasibility of developing a nationwide network of hydrogen refueling infrastructure to help convert the long-haul, heavy-duty (LHHD) truck fleet from diesel fuel to hydrogen. Diesel is currently the main fuel used to transport freight over the road.
The team carried out a technical analysis to estimate the number of hydrogen refueling stations needed to convert the fleet of LHHD vehicles to hydrogen. They also did an economic analysis to evaluate the final selling price of hydrogen.
According to Yaici, “Results from the techno-economic study showed that hydrogen prices can vary significantly.” On average, they found hydrogen to cost 239 percent more than diesel due to the diverse costs of the various production methods for hydrogen. Hydrogen also has about 2.6 times the energy density of diesel fuel by weight, which serves as an equalizer. Energy density by volume is another matter because at the operating temperature of trucks, diesel is a liquid while hydrogen is a gas, giving it a lower energy density by volume if it is not liquefied by cryogenics. Regardless of the hydrogen production method, the process can occur at a central facility or locally where it is needed.
“A central system can increase the scope of production by producing hydrogen at large facilities that take advantage of bulk processes to reduce costs per unit,” Yaici said. “Nevertheless, hydrogen must be transported to its destination via pipeline or trucks. This will increase the operational fees. A distributed process, one where hydrogen is produced on demand locally, eliminates the need to deliver hydrogen over long distances.”
Hydrogen stations are built to refuel vehicles in similar manner to those used by diesel stations. Each refueling station will consist of a compressor, storage tanks, and at least one dispenser. Yaici pointed out the versatility of the feasibility study. “The methodology and models proposed can be used for studying the techno-economics of hydrogen refueling stations in any country and for other types of ICE fleets or fuel cell vehicles,” he said.
This includes refueling stations for vehicle fleets powered by renewable fuels such as renewable natural gas or biofuels. Engineering analyses such as this one will help shape the future of transportation.
Tom Gibson, P.E. is a consulting mechanical engineer specializing in machine design, sustainability, and recycling based in Sugar Grove, Va.